Research On A New Non Electric Quantity Protection Strategy Of Oil Immersed Transformers

The oil immersed transformer represents a key device widely used in a modern power system.Its internal fault often causes its outage,or even a damage accident,which directly threatens the security of power system operation.At present,there are many methods to detect the faults,such as dissolved gas detection in oil,partial discharge detection and vibration detection.However,in the working conditions of high temperature,high pressure and oil seal,it is necessary to realize the fault protection of a transformer due to the bad working environment,strong noise and other factors.It is still a challenge for the existing methods to realize fast and accurate transformer fault detection and identification.The thesis starts with five parts,i.e.,electromagnetic transient modeling of transient electric quantity and non-electric quantity characteristics of oil immersed transformer,modeling of temperature field and calculation of hot spot temperature,turn to turn fault protection based on leakage flux characteristics,time scale analysis of non-electric quantity under an internal fault,and non-electric quantity protection scheme based on time sequence coordination.Based on the non-electric information,the inrush current and turn to turn fault can be identified.The main work of this thesis is as follows:First of all,starting from the structure of transformer,the composition structure and types of a transformer are analyzed.Then the equivalent circuit model of different magnetic circuit structures is constructed,which mainly relies on the dual change principle between different magnetic circuits and electric circuits;equivalent circuit models with different structures are next studied,including the winding resistance model,capacitance model and leakage inductance model.Finally,the wall model of the caier circuit transformer is constructed.Based on the transformer winding model and the core circuit topology,this thesis constructs the transformer electromagnetic transient model considering the hysteresis effect.The model mainly includes three-phase three column,three-phase five column and three-phase shell types.Secondly,based on the analysis of the heating and cooling process of the transformer,the lumped heat capacity and non-linear thermal resistance are defined by the principles of heat transfer,thermoelectric analogy method and finite element method.Considering the change of oil viscosity with temperature,the internal heat transfer process of the transformer is equivalent to the dynamic heat path calculation model.Thirdly,in view of the shortcomings of the prior art,the leakage flux distribution of the oil immersed transformer before and after the fault is recorded and compared from the distribution of the leakage flux under the turn to turn fault.The change of transverse flux leakage density amplitude and the symmetry of flux leakage are used as the protection criteria,and then the flux amplitude and symmetry are combined to identify the inrush current.The oil immersed transformer turn to turn fault protection method based on the characteristics of flux leakage is obtained.At last,temperature,magnetic flux leakage and gas flow rate are proposed as the main contents of non-electric quantity protection research.The principle of non-electric quantity and electric quantity protection is organically integrated from the protection object,time scale,action time and other aspects.Finally,the logic structure of integration of electric quantity and non-electric quantity protection of transformer based on time sequence coordination is obtained.